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Once we understand the concept of atomic mass – the relative mass of an individual atom – we can extend this idea to larger chemical entities: molecules and ionic compounds. This leads us to two important quantitative measures: molecular mass and formula unit mass.

1. Molecular Mass:

The molecular mass of a substance is simply the sum of the atomic masses of all the atoms present in a molecule of that substance. It represents the relative mass of a molecule expressed in atomic mass units (u). This calculation is straightforward: you identify all the atoms in the molecule, find their respective atomic masses from the periodic table (or a given table), and add them up, taking into account the number of each type of atom.

Example 1: Calculate the relative molecular mass of water (H₂O).

• Atomic mass of Hydrogen (H) = 1 u
• Atomic mass of Oxygen (O) = 16 u
• A water molecule has two hydrogen atoms and one oxygen atom.
• Molecular mass of H₂O=(2×Atomic mass of H)+(1×Atomic mass of O)
• Molecular mass of H₂O=(2×1 u)+(1×16 u)
• Molecular mass of H₂O=2 u+16 u=18 u

Example 2: Calculate the molecular mass of nitric acid (HNO₃).

• Atomic mass of Hydrogen (H) = 1 u
• Atomic mass of Nitrogen (N) = 14 u
• Atomic mass of Oxygen (O) = 16 u
• A nitric acid molecule has one hydrogen atom, one nitrogen atom, and three oxygen atoms.
• Molecular mass of HNO₃=(1×Atomic mass of H)+(1×Atomic mass of N)+(3×Atomic mass of O)
• Molecular mass of HNO₃=(1×1 u)+(1×14 u)+(3×16 u)
• Molecular mass of HNO₃=1 u+14 u+48 u=63 u

2. Formula Unit Mass:

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. It is calculated in precisely the same manner as molecular mass. The only distinction is in the terminology: we use the term "formula unit" specifically for those substances whose constituent particles are ions (ionic compounds), rather than discrete molecules.

Example: Calculate the formula unit mass of sodium chloride (NaCl).

• Atomic mass of Sodium (Na) = 23 u
• Atomic mass of Chlorine (Cl) = 35.5 u
• A sodium chloride formula unit has one sodium ion and one chloride ion.
• Formula unit mass of NaCl = (1×Atomic mass of Na)+(1×Atomic mass of Cl)
• Formula unit mass of NaCl = (1×23 u)+(1×35.5 u)
• Formula unit mass of NaCl = 23 u+35.5 u=58.5 u

Example: Calculate the formula unit mass of calcium chloride (CaCl₂).

• Atomic mass of Calcium (Ca) = 40 u
• Atomic mass of Chlorine (Cl) = 35.5 u
• A calcium chloride formula unit has one calcium ion and two chloride ions.
• Formula unit mass of CaCl₂=(1×Atomic mass of Ca)+(2×Atomic mass of Cl)
• Formula unit mass of CaCl₂=(1×40 u)+(2×35.5 u)
• Formula unit mass of CaCl₂=40 u+71 u=111 u

Both molecular mass and formula unit mass are crucial for understanding the quantitative relationships in chemical reactions and for performing calculations involving moles and stoichiometry, which are fundamental to practical chemistry.

Question for You: Calculate the molecular masses of H₂, O₂, Cl₂, and CO₂. (Atomic masses: H=1u, O=16u, Cl=35.5u, C=12u)